Inadvertent perioperative hypothermia complicates a large percentage of surgical procedures and is related to multiple factors. Strictly regulated in normal conditions (± 0.2°C), the core body temperature of an anaesthetised patient may fall by as much as 6°C, while a 2°C decrease is very common. This is due to a combination of anaesthesia-related impairment of the central thermoregulatory control and a cool operating room temperature, which, when superimposed on insufficient insulation and a failure to actively warm the patient, may result in profound temperature disturbances. As a result, prolonged wound healing, increased risk of wound infection, a higher rate of cardiac morbidity, and greater intraoperative blood loss and postoperative blood transfusion requirements may occur. The reasons for this are said to include underlying changes in microcirculation, coagulation, immunology and an increase in the duration of action of most anaesthesia medications. As effective methods have been available for a number of years now, it is currently indicated to maintain intraoperative normothermia in order to minimise procedure-related risk and improve patient comfort.
Inadvertent perioperative hypothermia is a common but preventable complication of perioperative procedures, which is associated with poor outcomes for patients. Inadvertent perioperative hypothermia should be distinguished from the deliberate induction of hypothermia for medical reasons, which is not covered by this guideline. In this guideline, hypothermia is defined as a patient core temperature of below 36.0°C. Hereafter, 'temperature' is used to denote core temperature. Adult surgical patients are at risk of developing hypothermia at any stage of the perioperative pathway. In the guideline, the perioperative pathway is divided into three phases: the preoperative phase is defined as the 1 hour before induction of anaesthesia (when the patient is prepared for surgery on the ward or in the emergency department), the intraoperative phase is defined as totalanaesthesia time, and the postoperative phase is defined as the 24 hours after entry into the recovery area in the theatre suite (which will include transfer to and time spent on the ward). The phrase 'comfortably warm' is used in recommendations relating to both the preoperative and postoperative phases, and refers to the expected normal temperature range of adult patients (between 36.5°C and 37.5°C).
Most clinically available thermometers accurately report the temperature of whatever tissue is being measured. The difficulty is that no reliably core-temperature-measuring sites are completely noninvasive and easy to use-especially in patients not undergoing general anesthesia. Nonetheless, temperature can be reliably measured in most patients. Body temperature should be measured in patients undergoing generalanesthesia exceeding 30 min in duration and in patients undergoing major operations during neuraxial anesthesia. Core body temperature is normally tightly regulated. All general anesthetics produce a profound dose-dependent reduction in the core temperature, triggering cold defenses, including arteriovenous shunt vasoconstriction and shivering. Anesthetic-induced impairment of normal thermoregulatory control, with the resulting core-to-peripheral redistribution of body heat, is the primary cause of hypothermia in most patients. Neuraxial anesthesia also impairs thermoregulatory control, although to a lesser extent than does general anesthesia. Prolonged epidural analgesia is associated with hyperthermia whose cause remains unknown.